SANAA, YEMEN - Police opened fire and used tear gas on anti-government demonstrators in the Yemeni capital, Sanaa, late Tuesday, injuring at least 50 people, four of them critically, doctors and witnesses said.

The violence erupted about 11 p.m. when demonstrators defied security forces by trying to take a tent through a line of checkpoints into a camp set up by opposition supporters at the entrance to Sanaa University. According to witnesses, police used tear gas, then fired in the air before shooting at the crowd. Ten people were shot, one of them in the face, according to a doctor at the scene.

A government statement said that police found on the demonstrators a stash of fully automatic rifles hidden inside thick blankets and camping equipment. It said that the demonstrators resisted arrest and that a gunfight then broke out in which dozens of suspected militants, policemen and bystanders were injured.

There have been daily anti-government demonstrations in Sanaa and other cities around the country since Egyptian President Hosni Mubarak's ouster Feb. 11. During the past weeks, 27 have been killed in the unrest, according to international rights groups.

In recent days, security has been stepped up in Sanaa. Military vehicles with mounted machine guns are in place at key sites, and water cannons have been deployed. President Ali Abdullah Saleh has said he will step down when his term ends in 2013 but has vowed to defend his government "with every drop of blood."

At an improvised medical center on the grounds of a mosque next to the university, volunteer doctors administered IV drips and treated bullet wounds. Ahmed Mohammed Saleh had a gaping bullet wound in his left hand and blood spattered across his face. As he was X-rayed, he said he had been shot by a group of 20 to 30 policemen as he emerged from his tent on Hurriya Street, on the edge of the protesters' camp.

Slumped on the floor nearby was Ahmed Noman, a 25-year-old demonstrator with a gunshot wound above his knee. He said he was in his tent when soldiers opened fire at him. Asked whether he would return to the demonstrations the next day, he said, "I'm going to go back in there right now."

Abdulrahman Ghalan, one of the volunteer doctors at the center, said that in addition to people with gunshot wounds, he had seen dozens who had been tear-gassed.

Outside, rows of men lay on the ground as oxygen was administered. Ashraf Mohammed Abdullah, a 22-year-old from Taiz, had been tear-gassed and shot in the legs - by police in a car on Hurriya Street, he said. He added that he would continue protesting as soon as he could, citing his frustration with unemployment and corruption.

At the scene of the shooting, the air was spicy with tear gas. About 20 police officers stood holding guns and clubs. "It was because of the tents," said one, speaking on the condition of anonymity. "They aren't allowed in here with tents."

At the medical center, blood dripped onto the dusty ground from the back of an ambulance. "They want to destroy the peace" of the demonstrations, said Mohammed Iryani, 20, a student, referring to the police and security forces. "But we will stay here until the government steps down."